One side of the bottom layer of a
triangular pyramid has 12 balls. How many
are there in the whole pyramid?
Note that the pyramid is equilateral and
solid.
a) 286 b) 364
c) 504 d) 164

• There are total 364 balls.
  
  As there are 12 balls along one side, it means that there are 12 layers of balls. The top most layer has 1 ball. The second layer has 3 (1+2) balls. The third layer has 6 (1+2+3) balls. The fourth layer has 10 (1+2+3+4) balls. The fifth layer has 15 (1+2+3+4+5) balls. Similarly, there are 21, 28, 36, 45, 55, 66 and 78 balls in the remaining layers.
  
  Hence, the total number of balls are
  = 1 + 3 + 6 + 10 + 15 + 21 +28 + 36 + 45 + 55 + 66 + 78
  = 364 balls
• 9 years agoHelpfull: Yes(47) No(1)
• b)364 answer
• 9 years agoHelpfull: Yes(3) No(1)
• answer is coming 650 but ther isnt the option...........
  coz every mat will contain balls layer by layer,since pyramid is in 3D
  
  12*12+11*11+10*10+9*9+8*8+7*7+6*6+5*5+4*4+3*3+2*2+1=650
  
• 9 years agoHelpfull: Yes(1) No(9)
• answer- 364
  
• 8 years agoHelpfull: Yes(0) No(1)